Accompanying the increasingly higher levels of integration of semiconductor devices in the semiconductor industry in recent years, there is a need for fine patterns that exceed the transfer limitations of conventional photolithographic methods using ultraviolet light. Extreme ultraviolet (EUV) lithography is considered to be promising as an exposure technology that uses EUV light to enable the formation of such fine patterns. Here, EUV light refers to light in the wavelength band of the soft X-ray region or vacuum ultraviolet region, and more specifically, light having a wavelength of about 0.2 nm to 100 nm. Reflective masks have been proposed as transfer masks for use in EUV lithography. Such reflective masks have a multilayer reflective film that reflects exposure light formed on a substrate, and an absorber film that absorbs exposure light formed in a pattern on the multilayer reflective film.
The reflective mask is fabricated from a substrate, a multilayer reflective film formed on the substrate, and a reflective mask blank having an absorber film formed on the multilayer reflective film, by forming an absorber film pattern by photolithography and the like.
As described above, due to the growing demand for miniaturization in the lithography process, significant problems are being encountered in the lithography process. One of these is the problem relating to defect information of mask blank substrates and substrates with a multilayer reflective film and the like used in the lithography process.
Mask blank substrates are being required to have even higher smoothness from the viewpoints of improving defect quality accompanying the miniaturization of patterns in recent years and the optical properties required of transfer masks.
In addition, substrates with a multilayer reflective film are also being required to have even higher smoothness from the viewpoints of improving defect quality accompanying the miniaturization of patterns in recent years and the optical properties required of transfer masks. Multilayer reflective films are formed by alternately laminating layers having a high refractive index and layers having a low refractive index on the surface of a mask blank substrate. Each of these layers is typically formed by sputtering using sputtering targets composed of the materials that form these layers.
Ion beam sputtering is preferably carried out as the sputtering method from the viewpoint of being resistant to contamination by impurities present in the multilayer reflective film as a result of not requiring the generation of plasma by electrical discharge, and having an independent ion source thereby making setting of conditions comparatively easy. In the case of using ion beam sputtering, from the viewpoint of the smoothness and surface uniformity of each layer formed, the high refractive index layer and low refractive index layer are deposited by allowing sputtered particles to reach the target at a large angle with respect to the normal (line perpendicular to a main surface of the mask blank substrate) of a main surface of the mask blank substrate, or in other words, at an angle diagonal or nearly parallel to a main surface of the substrate.
Patent Literature 1 describes a technology for manufacturing a substrate with a multilayer reflective film using such a method in which, when depositing a multilayer reflective film of a reflective mask blank for EUV lithography on a substrate, ion beam sputtering is carried out by maintaining the absolute value of an angle α formed between the normal of the substrate and sputtered particles entering the substrate such that 35 degrees≤α≤80 degrees while rotating the substrate about the central axis thereof.